How To Identify an Unknown Organic Compound

Alternatively, the compound can be determined by analysis of its spectrum from laboratory equipment.

This consists of gathering the following information: boiling point (if a liquid unknown), melting point (if a solid unknown), IR spectrum, mass spectrum, PMR spectrum, decoupled CMR spectrum (optional).

When analyzing the IR spectrum, remember that the absence of a band is just as informative as the presence of a band. For example, if OH band is absent, you can eliminate the possibility of an alcohol or carboxylic acid functional group.

For a given functional group, more than one band is used to identify that functional group, and they all have to be present. For example, to identify an alcohol, both the OH and the C-O bands must be present.

Using the mass spectrum, identify the presence or absence of Cl or Br in the compound, based on isotopic abundances of these halogens. Try to use the molecular ion peaks, not the base peak, to look for a 3:1 abundance of the M:M+2 peaks (Cl present) or 1:1 abundance of the M:M+2 peaks (Br present). The molecular ion peak is the signal of highest mass which is also of appreciable abundance, and gives the molecular weight of the compound. While some mass spectra will not show a molecular ion peak, the majority of the spectra will have this peak and it is reasonably obvious. Alcohols and aldehydes often will have a M-1 peak or an M-18 peak instead of a molecular ion peak. Also, if the molecular weight is an odd number (not even), then nitrogen is probably found in the compound.

Using the boiling point or melting point and the functional group of highest priority, set up a list of possible compounds from the CRC Handbook of Tables for Organic Compound Identification, or your organic lab text. Consider compounds in the range of ±10 of the measured boiling point or melting point.

Start narrowing down this list based on molecular weight and presence or absence of other functional groups.

Use the NMR and CMR to eliminate any further possibilities. Remember that the decoupled CMR will tell you how many carbon environments are present in the compound.